Such valves, in the form of proportional pressure regulating valves, are widely used for mobile working devices for the electro-hydraulic control of clutches.
These clutches must be filled regularly with an operating fluid in the form of oil when activated initially until their friction surfaces reach the point of contact to be able to act as a clutch. To this end, spring forces must be overcome in the clutch. The pressures created by spring forces are often very low (less than 2 bar). A further increase in the clutch pressure then results in normal forces on the clutch linings, which can ultimately transmit the torque through frictional forces.
These low spring forces result in problems when switching off the clutches because, to move the respective clutch disc away from the assignable contact surface, only the pressure caused by these spring forces is available to produce the oil flow through the proportional pressure regulating valves.
There is therefore a requirement for these valves that the pressure loss in the flow from the working port to the tank port should be extremely small because that is the only way to ensure fast and safe switching off of the clutch.
In the current prior art, the largest possible flow cross-section is opened in directly controlled proportional pressure regulating valves through the use of magnets with large working strokes. At the same time, this opening requires as much magnetic force as possible in order not to let the ratio between magnetic force and flow force become too unfavorable. Large and expensive actuator devices in the form of actuating magnets are therefore required.
If one were now to overcome these disadvantages by using smaller, and thus more economical actuating magnets with the same force, which is within the average skill of a person skilled in the art in the field of valve and clutch technology, decreasing the length of the linear force-stroke range would be necessary. If one does not intend to reduce the full opening cross-section for the fast emptying of the fluid medium from the clutch, one must not shorten the actual valve stroke of the valve piston. This situation in turn inevitably results in the spring forces holding the actuating magnet in its non-linear force range when the valve is switched on, requiring a large magnet current for “breaking away the magnet” from the end position.
At the moment, however, where the magnetic force becomes greater than the spring force, one moves on the strongly ascending branch of the characteristic force-stroke line, and the balance between magnetic force and spring force would be lost in favor of the magnetic force. On the PI characteristic line of the valve, this situation would be noticeable from a start-up jump with the result that, upon releasing the clutch, the vehicle starts with a jump, which is particularly unacceptable for safety reasons for mobile working devices.